Simple vs Compound Interest: Key Differences
Both grow your money — but compound interest grows it exponentially. Here's a clear, beginner-friendly comparison with formulas, real numbers, and a 40-year side-by-side table.
See the difference for your own numbers
Try our free compound interest calculator to model your savings or investments. Adjust the rate, time, and contributions and watch the growth curve update instantly.
Open Compound Interest CalculatorSimple interest
Simple interest is calculated only on the original principal. It does not earn interest on previously earned interest. The formula is:
Example: $10,000 at 5% simple interest for 20 years = $10,000 × (1 + 0.05 × 20) = $20,000 ($10,000 of interest earned).
Compound interest
Compound interest is calculated on the principal and all previously earned interest. The formula is:
Example: $10,000 at 5% compounded monthly for 20 years ≈ $27,126 — that's $7,126 more than simple interest, with no extra deposits. For a full breakdown, see the compound interest formula explained.
Side-by-side comparison
Here's how $10,000 at 5% grows under each method over time:
| Years | Simple Interest (5%) | Compound Interest (5%) | Difference |
|---|---|---|---|
| 5 | $12,500 | $12,834 | +$334 |
| 10 | $15,000 | $16,470 | +$1,470 |
| 20 | $20,000 | $27,126 | +$7,126 |
| 30 | $25,000 | $44,677 | +$19,677 |
| 40 | $30,000 | $73,584 | +$43,584 |
All examples use $10,000 initial principal, monthly compounding.
Key takeaways
- Compound interest accelerates over time. The longer the time horizon, the bigger the gap.
- Simple interest is linear. You earn the same dollar amount every year.
- Compounding frequency matters. Monthly beats annual; daily beats monthly — by a little each year, by a lot over decades.
- It cuts both ways. Compounding helps savers but hurts borrowers carrying credit-card balances.
Where you find each in real life
- Simple interest: Most car loans, some personal loans, certain bonds, Treasury bills.
- Compound interest: Savings accounts, CDs, mortgages, credit cards, index funds, 401(k)s, IRAs.
Want the deeper story on why compounding is so powerful? Read how compound interest works.
See the difference for your own numbers
Try our free compound interest calculator to model your savings or investments. Adjust the rate, time, and contributions and watch the growth curve update instantly.
Open Compound Interest CalculatorFrequently Asked Questions
What is the difference between simple and compound interest?
Simple interest is calculated only on your original principal — it's a flat percentage every year. Compound interest is calculated on your principal plus all the interest you've already earned, so growth accelerates over time.
Which is better, simple or compound interest?
It depends which side you're on. As a saver or investor, compound interest is dramatically better because your money grows faster. As a borrower, simple interest is friendlier because your debt doesn't snowball — that's why many car loans use simple interest.
Where is simple interest used in real life?
Simple interest is common on most car loans, some short-term personal loans, and certain bonds. It's also used on Treasury bills and for many promissory notes between individuals.
Where is compound interest used?
Compound interest is used on savings accounts, CDs, money-market accounts, mortgages, credit cards, student loans, and almost all long-term investment products including index funds, retirement accounts, and dividend-reinvestment plans.
How big is the difference over 30 years?
On $10,000 at 5% for 30 years, simple interest yields $25,000 total ($15,000 in interest). Compound interest (monthly) yields about $44,677 total — almost 80% more interest earned with no extra effort.
Can I see this for my own numbers?
Yes — our compound interest calculator runs both scenarios for you. You can adjust the principal, rate, time, and contribution to see exactly how much compounding adds to your bottom line.